There has been known an asymmetric hollow fiber membrane for gas separation produced using an aromatic polyimide prepared from a tetracarboxylic acid component containing biphenyl tetracarboxylic acid, pyromellitic acid and 4,4′-(hexafluoroisopropylidene)diphthalic acid, and a diamine component having an intramolecular sulfonic group.
As a hollow fiber membrane having such a composition, Patent Reference No. 1 (Japanese laid-open patent publication No. 1993-68859) has described a gas separation membrane exhibiting higher gas permselectivity and having excellent mechanical properties, but a separation coefficient α (PO2/P′N2) as a representative element of gas permselectivity is 4.7 to 4.9, which must be further improved.
Patent Reference No. 2 (Japanese laid-open patent publication No. 1994-254367) has described an excellent gas separation membrane having a high separation coefficient α. This gas separation membrane is suitable for producing a nitrogen-rich air and an oxygen-rich air and has a separation coefficient α(PO2/P′N2) of 5.3 or more. The membrane, however, frequently has a low permeation rate P′O2 of 4.4 or less, and thus, when it is used in a module, it causes deterioration in performance due to variation in a permeation rate P′O2 or P′N2, and when P′O2 or P′N2 is low, a too large membrane area is required, leading to disadvantage that a module cannot be efficiently designed or produced. Patent Reference No. 2 has described an example with a large permeation rate P′O2 (Example 1), but has not described mechanical properties. A membrane in this example has a low density probably because a dope solution for spinning has a low concentration and a relatively lower molecular weight. As a result, a large permeation rate P′O2 can be obtained, whereas mechanical strength is reduced. In particular, since the membrane is processed at a high temperature of 330° C. for achieving performance of α≧5.3, it further has insufficient mechanical properties such as elongation at break. When the elongation at break is insufficient, yarn break during producing a separation module by bundling hollow fiber membranes, or deterioration or variation in performance is caused.